Body is long and cylindrical, except for the anterior branchial part, which is broader and flattened.
Prostomium is triangular with two eyes and two nuchal slits. Antennae absent.
Notopodial postchaetal lobes on the first chaetiger short, finger-like becoming longer and cirriform in the branchial region, behind the gills very short again, posteriorly becoming longer and thinner.
Modified chaetae of two kinds, in notopodia only: 2-4 forked chaetae with a long and a short branch, starting at notopodium 3-10, and 1-2 spines with thin subdistal prolongation, starting at notopodium 17 or 18 (C. armatus-detail).
Up to 19 pairs of long gills dorsally, starting at chaetiger 4.
Pygidium with three long cirri.
Up to 35 mm for 140 segments.
Living animal pale reddish. In alcohol colourless.
On sand or mud; eulittoral to 1200 m.
Eastern Atlantic between Ireland and Morocco, Mediterranean, English Channel, North Sea.